[PDB] Don't crash on /debug:fastlink PDBs.

Apparently support for /debug:fastlink PDBs isn't part of the
DIA SDK (!), and it was causing llvm-pdbdump to crash because
we weren't checking for a null pointer return value.  This
manifests when calling findChildren on the IDiaSymbol, and
it returns E_NOTIMPL.

llvm-svn: 304982
diff --git a/llvm/lib/DebugInfo/PDB/DIA/DIARawSymbol.cpp b/llvm/lib/DebugInfo/PDB/DIA/DIARawSymbol.cpp
index 4e2474c..0b48a36 100644
--- a/llvm/lib/DebugInfo/PDB/DIA/DIARawSymbol.cpp
+++ b/llvm/lib/DebugInfo/PDB/DIA/DIARawSymbol.cpp
@@ -372,8 +372,11 @@
   enum SymTagEnum EnumVal = static_cast<enum SymTagEnum>(Type);
 
   CComPtr<IDiaEnumSymbols> DiaEnumerator;
-  if (S_OK != Symbol->findChildrenEx(EnumVal, nullptr, nsNone, &DiaEnumerator))
-    return nullptr;
+  if (S_OK !=
+      Symbol->findChildrenEx(EnumVal, nullptr, nsNone, &DiaEnumerator)) {
+    if (S_OK != Symbol->findChildren(EnumVal, nullptr, nsNone, &DiaEnumerator))
+      return nullptr;
+  }
 
   return llvm::make_unique<DIAEnumSymbols>(Session, DiaEnumerator);
 }